Generally, separation membranes are made from various inorganic or organic materials, including ceramics, metals and polymers. For example, ceramic materials possessing oxide ion conductivity are suitable to cause selective permeation of oxygen at high temperatures, such as temperatures of about 500° C. or more. Membranes comprising at least a layer of said ceramic materials are therefore suitable to separate oxygen from oxygen containing gas mixtures.
More specifically, it has been suggested to apply electrodes to both sides of a ceramic membrane structure being based on electrolytic material and to connect said electrodes externally. On one side of the membrane, the oxygen partial pressure is during use lower than on the other side of the membrane. In said configuration, oxygen molecules at the side with the higher oxygen partial pressure accept electrons, split and become oxygen ions, which diffuse through the membrane to the opposite electrode, where they discharge, and leave the membrane, either as oxygen molecules or, in the case of a combustible gas being present, as part of a combustion product. The electrons are transferred back via the external circuit to the first electrode. As a result, oxygen is continuously separated from the gas at the side of the membrane which has the higher oxygen partial pressure.
The above described membranes are also suitable for partial oxidation processes, for instance oxidation of methane gas in order to produce syngas, i.e. a mixture of CO and H2. Syngas is an important intermediate product in the production of methanol, ammonia, or synthetic diesel.
Some oxygen ion conductors also exhibit electron conductivity, referred to as electron—oxide ion mixed conductors, or just mixed conductors. If the electronic conductivity is not sufficiently high, dual conducting mixtures may be prepared by mixing an ion-conducting material with an electronically conducting material to form a composite, multi-component, non-single phase material.
Additionally, membranes can be used to separate hydrogen. In this case, the membrane material must be a proton conductor. Hydrogen can serve as a clean fuel for powering many devices ranging from large turbine engines in integrated gasification combined cycle electric power plants, to small fuel cells. Hydrogen can also power automobiles, and large quantities are used in petroleum refining.
In case of syngas production, the above described ceramic membranes are exposed to extreme conditions. The opposite sides of the membrane are simultaneously exposed to a highly oxidizing and a highly reducing atmosphere, respectively, at high temperatures. Also the thermal and chemical expansion of the membrane at high temperatures (and low pO2) might result in stress in the membrane and in the other parts of the apparatus containing said membrane. The membranes therefore need chemical stability with respect to decomposition and should further exhibit low expansion on reduction.
Oxygen separation membranes may also be operated at high pO2 where the driving force for the flux of oxygen is created by having a high absolute pressure difference over the membrane. In this case, the chemically environment is more benign, but the mechanical loads introduced by the pressure differences are severe and a structurally robust membrane design is necessary.
The following Table lists some of the proposed materials for oxygen separation together with some of their properties.
TABLE 1Oxide ion conductivity and pO2 stabilitylimits of membrane candidate materialsestimated de-σO (S/m)σO (S/m)compositon(1073 K)(1273 K)pO2 (atm)La0.6Sr0.4FeO3−δ1 [1]20 [1]10−17(1273 K)10−14(1473 K)La0.6Sr0.4Co0.2Fe0.8O3−δ4 [3]20 [3]10−7 (1273 K)La0.6Sr0.4CoO3−δ6 [4]40 [4]10−7 (1273 K)Ba0.5Sr0.5FeO3−δ>4 [5] >8 [5]10−17(1273 K)Ba0.5Sr0.5Co0.8Fe0.2O3−δ>27 [5] >47 [5] 10−7 (1273 K)Ce0.9Gd0.1O1.95−δ6 [6]16 [6]—Ce0.8Gd0.2O1.9−δ6 [6], 20 [7]16 [6], 25 [7]—Y0.16Zr0.84O1.9210—References in Table 1:[1] M. Søgaard, P. V. Hendriksen, M. Mogensen, “Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum ferrite”, J. Solid State Chem 180 (2007) 1489-1503.[2] T. Nakamura, G. Petzow, L. J. Gauckler, “Stability of the perovskite phase LaBO3 (B = V, Cr, Mn, Fe, Co, Ni) in a reducing atmosphere i. experimental results”, Materials Research Bulletin 14 (1979) 649-659.[3] B. Dalslet, M. Søgaard, P. V. Hendriksen, “Determination of oxygen transport properties from flux and driving force measurements using an oxygen pump and an electrolyte probe”, J. Electrochem. Soc., to be published.[4] M. Søgaard, P. V. Hendriksen, M. Mogensen, F. W. Poulsen, E. Skou, “Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite”, Solid State Ionics 177 (2006) 3285-3296.[5] Z. Chen, R. Ran, W. Zhou, Z. Shao, S. Liu, “Assessment of Ba0.5Sr0.5Co1−yFeyO3−δ (y = 0.0-1.0) for prospective application as cathode for it-SOFCs or oxygen permeating membrane”, Electrochimica Acta 52 (2007) 7343-7351.[6] S. Wang, H. Inaba, H. Tagawa, M. Dokiya, T. Hashimoto, “Nonstoichiometry of Ce0.9Gd0.1O1.95−x”, Solid State Ionics 107 (1998) 73-79.[7] N. Sammes, Z. Cai, “Ionic conductivity of ceria/yttria stabilized zirconia electrolyte materials”, Solid State Ionics 100 (1997) 39-44.
Especially flourite and perovskite structured metal oxide materials offer a number of candidates for good oxygen separation membranes. Table 1 lists the oxygen ion conductivity, σo of these materials as well as the pO2 of decomposition at various temperatures (the pO2 of decomposition is estimated as the pO2 of decomposition of LaCoO3 for the Co containing perovskites, and the pO2 of decomposition of LaFeO3 for the Fe containing perovskites). The other listed materials in Table 1 are stable in the pO2 range required for operating a syngas membrane.
As is evident from the Table, the Co-containing perovskites exhibit a high ionic conductivity. However, they do not possess sufficient thermodynamic stability for operating at low pO2, as is required for instance for production of synthesis gas in a membrane reactor.
On the other hand, of the materials possessing sufficient thermodynamic stability as required for syngas production, doped ceria possesses the highest ionic conductivity as compared to the above perovskite candidates.
The performance of a mixed conducting membrane will in general be limited by either the electronic or the ionic conductivity, whichever is lower. For the perovskite materials, the ionic conductivity is generally the limiting factor, whereas the electronic conductivity is the limiting factor for the fluorite materials. At high pO2 the performance of Ce0.9Gd0.1O1.95-δ and Ce0.8Gd0.2O1.9-δ will be limited by their electronic conductivity. It has been suggested to enhance the electronic conductivity by using Pr substitution rather than Gd substitution. However, in order to improve the performance of the membrane, for example for the syngas production, new materials are desired exhibiting a better balance of ionic and electronic conductivity to overcome the current limits as provided by the prior art.
U.S. Pat. No. 6,139,810 discloses a reactor comprising reaction tubes which comprise an oxygen selective ion transport membrane with an anode side, wherein said membrane is formed from a mixed conductor metal oxide, a heat transfer means formed from metal, and a reforming catalyst disposed about said anode side of said oxygen selective ion transport membrane.
WO-A1-01/09968 relates to mechanically strong, highly electronically conductive porous substrates for solid-state electrochemical devices. A gas separation device is disclosed comprising a first electrode comprising a metal and a second electrode comprising a ceramic material.
U.S. Pat. No. 6,033,632 relates to solid state gas-impermeable, ceramic membranes useful for promotion of oxidation-reduction reactions as well as for oxygen gas separation. The membranes are fabricated from a single-component material which exhibits both, electron conductivity and oxygen-ion conductivity. Said material has a brownmillerite structure with the general formula A2B2O5.
EP-A-0 766 330 discloses a solid multi-component membrane which comprises intimate, gas-impervious, multi-phase mixtures of an electronically-conductive phase and/or gas-impervious “single phase” mixed metal oxides having a perowskite structure and having both electron-conductive and oxygen ion-conductive properties.
U.S. Pat. No. 6,165,553 discloses a method of fabricating a dense ceramic membrane comprising:                providing a colloidal suspension of a ceramic powder;        providing a polymeric precursor comprising a polymer containing metal cations;        mixing the polymeric precursor together with the colloidal suspension;        applying the mixture to a membrane support to form a composite structure; and        heating the composite structure to form a dense membrane on the membrane support.        
US-A-2005/0142053 relates to a composite-type mixed oxygen ion and electronic conductor, characterized in that its oxygen ion conductive phase consists of gadoliniumdoped cerium oxide, and its electronic conductive phase consists of a spinel-type ferrite.
U.S. Pat. No. 6,541,159 discloses an oxygen separating membrane, comprising a backbone having a first surface and a second surface and an array of interconnected pores extending therebetween; a hydroxide ion conductor extending through said pores from said first surface to said second surface; and an electrical conductor extending through said pores from said first surface to said second surface, said electrical conductor being discrete from said ion conductor.
However, the membrane structures suggested in the prior art are insufficient in chemical and mechanical stability and reliability and/or do not provide the required high performance, and are further expensive in production, thus preventing mass production. The suggested membranes do not result in membranes having a good balance of ionic and electronic conductivity, limiting the membrane efficiency due to the inherent limit of either the electrical or ionic conductivity of the employed materials. On the other hand, the suggested materials showing a promising balance are chemically unstable structures or otherwise not suitable for membrane mass production, as the membranes have a very short life time or they are expensive to manufacture.